1. Field of the Invention
The present invention relates generally to multiple voting solenoid-operated valve devices for testing and controlling process and fluid transport systems, and more particularly to a variable function voting solenoid-operated valve apparatus having air-to-move fail last position actuators to direct the flow of a fluid media.
2. Background of the Invention
Modern process or manufacturing plants consist of innumerable individual components. These components are integrated to form operational systems controlled by instrumentation and control systems consisting of a variety of sensors and controllers. The operation and control systems serve not only to achieve desired process conditions and parameters, but also to allow a monitor (e.g., an operator or a processor) to safely modify or discontinue operation of all or part of the plant's systems and components if necessary in order to avoid circumstances that define a safety condition.
One previously known safety system operates by isolating or venting process fluids when an unsafe operating condition has been detected by the safety system. Depending on the particular operational parameters, isolation and/or venting of process fluid can be accomplished by actuating one or more solenoid-operated process valves.
Solenoid valves are essentially electro-mechanical devices that use a solenoid to control valve actuation. Previously known solenoid-operated process valves generally include a wire coil and a movable plunger that seats against the coil; when electrical current is supplied to the solenoid coil, an actuating magnetic field is induced that acts upon the plunger. If an attached positioner detects a difference in position between the plunger's position and the control signal, the positioner sends a signal to move the plunger so that the correct position is achieved.
Those of ordinary skill in the art have found such systems deficient, however, primarily because of their dependence on electronic control signals and inexpensive mechanical parts. For example, should one or more of the coil, positioner or plunger fail, there is no way to accurately test whether the position control signal matches the actual solenoid position so that a change in state should be expected.
In other previously known devices, a pneumatic fluid supply is directed toward a process valve actuator through a solenoid-operated valve in order to vent process pressure when a predefined safety condition is detected. Typically, a pneumatic pressure source is supplied to one side of the actuator, while accumulated pressure is vented on the other side. If an associated solenoid changes state or position in a predefined manner, e.g., when a solenoid is de-energized by an operator or an associated logic control system, the plant system being tested is placed in an operational mode defined as a “safety action”.
Those of ordinary skill in the art will appreciate, however, that testing and maintenance of individual solenoid-operated valves should preferably occur without initiation of the safety action, so that there is no need to take system being monitored off-line during testing and maintenance. Thus, single solenoid valve configurations have in the past imposed a trade-off for operators between either discontinuing safety monitoring during testing and maintenance, or else risking false initiation of the safety action as a result of limited or incomplete testing and maintenance.
For example, a “1 out of 1” solenoid-operated valve configuration is previously known in which a single solenoid-operated valve activates the system's safety action by actuating process valves upon detection of an unsafe condition. Such configurations can achieve high plant safety availability when valve operation is regularly tested by first de-energizing the solenoid valve, and then monitoring a venting of the fluid or pneumatic supply through an exhaust body or manifold. Since the process valve and, ultimately, the process or fluid transport system (or some of the system's constituent components) may be adversely affected by such venting, testing of the solenoid-operated valve can only be performed under plant bypass conditions, wherein the fluid or pneumatic supply is allowed to pass directly to one side of the process valve actuator while pressure is vented on the other side of the actuator by means of a bypass valve. However, when the solenoid-operated valve is bypassed for testing, the safety action (actuation of the process valve) intended to avoid the unsafe condition cannot be initiated.
Thus, in practice, the overall safety availability performance of a 1 out of 1 solenoid-operated valve is limited by the percentage of operational time required in a bypass state for testing and maintenance. Moreover, such configurations can achieve only relatively low plant system reliability outside of testing and routine maintenance, since an unexpected component failure within the solenoid-operated valve, e.g., a coil failure or the like, will necessarily cause an inadvertent venting or isolation of the fluid or pneumatic supply, i.e., actuation of the process valve and initiation of the safety action.
A “1 out of 2” solenoid-operated valve configuration is also known wherein the correct functioning of one of a pair of solenoid-operated valves connected in operative association is required to actuate the process valve and initiate the safety action. Since only one of the solenoid-operated valves is required to actuate the process valve, relatively high plant safety availability is inherently provided.
Moreover, such configurations do not require the high testing frequency of the 1 out of 1 solenoid-operated valve system. However, routine testing and maintenance of the device are still required for ordinary safety applications. Similar to the 1 out of 1 solenoid valve, the 1 out of 2 configuration typically requires bypassing the system's safety action during testing. Accordingly, the device is incapable of actuating the process valve, and of isolating or venting the process fluid supply in response to an unsafe condition while the system is in bypass mode. Thus, the safety availability performance of the 1 out of 2 solenoid is limited by the percentage of operational time required for bypassing and testing. Moreover, since there are two discrete solenoids capable of initiating the safety action, a failure in a single solenoid-operated valve coil can lead to the inadvertent actuation of the process valve and isolation or venting of the process fluid.
A “2 out of 2” configuration has also been employed wherein both solenoid-operated valves must correctly function to actuate the process valve and initiate the safety action. Since both solenoid-operated valves must function properly, high plant system reliability is readily obtained. However, since the likelihood of individual component failure within the solenoid-operated valve system is effectively doubled (for example, both solenoid-operated valves must always function properly), the configuration suffers from relatively low safety availability unless it is frequently function-tested. Also, initiation of the safety action is prevented during testing and maintenance because the plant system must be bypassed for such functions, wherein the fluid or pneumatic supply is allowed to pass directly to the process valve or its constituent components by means of a bypass valve. When the solenoid-operated valves are bypassed for testing, the safety action (i.e., actuation of the process valve) intended to avoid the unsafe condition cannot be initiated. Moreover, the testing and maintenance cycle is generally time and manpower intensive since most of the known 2 out of 2 configuration is still tested manually. As with the previously discussed solenoid-operated valve system, therefore, the safety availability performance of the device is limited by the percentage of operational time required during bypassing and testing or maintenance.
A more sophisticated approach has involved a “2 out of 3” voting solenoid-operated valve system wherein two out of three solenoid-operated valves must operate properly to actuate the process valve and isolate or vent the process fluid. The configuration achieves high safety availability since only two of the three solenoid-operated valves must function to initiate a safety action, and high plant system reliability since two of the three solenoid-operated valves must experience a coil failure or the like for an inadvertent actuation of the process valve and isolation or venting of the process fluid supply to occur. The safety availability is also superior to the previously discussed solenoid systems insofar as the device can be tested or maintained without bypassing the safety action.
In practice, however, those of skill in the pertinent arts have found that the use of three solenoid-operated valves substantially increases the overall price and complexity of the system. Additional logic control system input and output points are also required relative to simpler configurations, and thus installation and operating expenses are correspondingly increased. In short, the high cost and relative complexity of the 2 out of 3 solenoid configuration have greatly limited its effective industrial utility.
Finally, elaborate “quad-voting” configuration have also been attempted wherein both high safety availability and high plant system reliability are reportedly achieved. However, the use of four solenoid-operated valves in a multiple voting configuration has been found to require an unusually large amount of space to accommodate its complex pneumatic tubing and logic control systems, and such complexity obviously increases the associated installation and maintenance expense. Moreover, many commercial operators of voting solenoid-operated valve systems have been found to disfavor the complex quad-voting configuration because of the elevated potential for testing and maintenance error associated therewith.
In view of the foregoing, it is clear there is a widespread need for a double acting variable function voting solenoid-operated valve apparatus having both a high safety availability and high plant system reliability that does not require a plant system to be bypassed during testing and maintenance, and especially for solenoid-operated valves having a redundant control system that can safely and accurately test and monitor a safety system.